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The synthesis and thermal properties of new shape-persistent macrocycles of different sizes decorated with intraannular alkyl chains are described. The alkyl chain length is in all cases sufficient to cross the rings and to fill their interior completely. The investigation of the thermal behavior has shown that the smaller cycles do not exhibit thermotropic mesophases. Single crystal x-ray analysis indicates that the anisotropy in these compounds is too small to describe them as plates rather than spheres. For the larger macrocycles it is shown that longer adaptable substituents decrease the phase transition temperatures compared to previously described structures.

Phase-contrast magnetic resonance (MR) imaging is a well-known but undervalued method of obtaining quantitative information on blood flow. Applications of this technique in cardiovascular MR imaging are expanding. According to the sequences available, phase-contrast measurement can be performed in a breath hold or during normal respiration. Prospective as well as retrospective gating techniques can be used. Common errors in phase-contrast imaging include mismatched encoding velocity, deviation of the imaging plane, inadequate temporal resolution, inadequate spatial resolution, accelerated flow and spatial misregistration, and phase offset errors. Flow measurements are most precise if the imaging plane is perpendicular to the vessel of interest and flow encoding is set to through-plane flow. The sequence should be repeated at least once, with a high encoding velocity used initially. If peak velocity has to be estimated, flow measurement is repeated with an adapted encoding velocity. The overall error of a phase-contrast flow measurement comprises errors during prescription as well as errors that occur during image analysis of the flow data. With phase-contrast imaging, the overall error in flow measurement can be reduced to less than 10%, an acceptable level of error for routine clinical use., (Copyright RSNA, 2002)